The use of pavement markings (e.g., paints, retroreflective elements, tapes, and raised pavement markings) to guide and direct motorists traveling along a roadway is well known. These pavement markings often are retroreflective so motorists can see the markings at night. However, when the roadway is wet, for example from rainfall, the pavement marking in turn becomes wet and often the retroreflective performance diminishes.
Retroreflection describes the mechanism where light incident on a surface is reflected so that much of the incident beam is directed back toward its source. When the surface of the pavement marking becomes wet, the optical elements (i.e., transparent, substantially spherical, glass or ceramic lenses) become coated with water, which typically reduces retroreflection. When optical elements become wetted or covered with water, the ratio of the refractive index at the exposed-lens surface changes which affects the light gathering.
Examples of retroreflective elements or aggregates known in the art include, but are not limited to, U.S. Pat. Nos. 3,252,376; 3,254,563; 4,983,458; 4,072,403; 4,652,172; 5,268,789; 5,750,191; 5,774,265; and 5,822,120. Many variations are known, but the retroreflective elements essentially have a core with optical elements embedded in the core surface. Some known embodiments also contain optical elements dispersed throughout the core. The core typically is regularly shaped e.g., spheres, tetrahedrons, discs, square tiles, etc. Retroreflective elements are advantageous because they can be embedded into inexpensive painted markings.
Retroreflective elements largely comprise polymeric cores or binders. A pigmented core or binder often serves as a diffuse reflector. This arrangement allows optical elements to be used on either horizontal or vertical surfaces. Other constructions have transparent optical elements comprising a specular reflector such as metallic silver. The metallic surface directs light back towards the source and a pigmented core is not necessary. Because of the geometry of the optics, a specular coated optical element would not be as effective if embedded in a pavement marking paint (a horizontal surface), and would be more highly effective if embedded in the vertical (i.e. generally up-right) edges of a retroreflective element.
Retroreflective elements can also be constructed having a ceramic core and glass optical elements with a metallic specular coating, (e.g., U.S. Pat. Nos. 3,043,196, 3,175,935, 3,556,637, 3,274,888, 3,486,952, EP 0,322,671). Ceramic retroreflective elements typically exhibit greater resistance to weathering and to wear, but often require substantially higher processing temperatures which increases cost.
Retroreflective elements can be formed by various methods. For example, drops of liquid resin can dropped into a bed of glass optical elements. The optical elements embed into the resin and then the resin hardens. (U.S. Pat. No. 3,254,563).
Another formation method is casting liquid resin mixed with glass optical elements into a desired shape and spraying the exposed surfaces with additional glass optical elements. The resin is then hardened. (U.S. Pat. No. 4,983,458).
Another method is calendering polymeric material through a set of rollers containing die-forming recesses. The optical elements are then attached to the bottom of the core with a transparent polymer binder. Specular film is applied by vacuum metallization. (U.S. Pat. Nos. 4,072,403, 4,652,172, 5,268,789).
U.S. Pat. No. 3,958,891 discloses skid-resistant or retroreflective elements manufactured by cutting or punching small disks from calendered tape (such as epoxy or polyurethane resin). The disks are then coated with a layer of resinous binder and a monolayer of optical elements. After the binder substantially sets, a further layer of binder and a monolayer of optical elements are applied. These steps are repeated until the desired coating of optical elements is obtained.
Another method of forming retroreflective elements is to extrude and pelletize cores and then place the cores in a bed of pre-heated optical elements, where the optical elements embed into the core (U.S. Pat. No. 5,750,191, Hachey et al.).
Each of these methods forms a retroreflective element having optical elements covering substantially all of the core surface area.
One means of reducing the cost of retroreflective elements without substantially affecting retroreflective performance, is to selectively place optical elements on vertical surfaces. The optical elements are relatively expensive, particularly the ceramic optical elements, thus limiting their placement to vertical surfaces where light is optimally retroreflected and foregoing placement on horizontal surfaces, is often desirable.
In the embossed pavement marking tape area, U.S. Pat. Nos. 5,227,221, 4,988,555, and 4,988,541 disclose pavement marking tapes having a patterned base sheet and selectively applying a bonding material to the protuberances so that the optical elements and/or skid-resistant particles are secured exclusively to the protuberances having bonding material where they are most effective. The optical elements and/or skid-resistant particles are substantially absent from the valleys where they make little contribution to the retroreflective performance or the skid resistance of the pavement marking. By selectively securing the optical elements and skid-resistant particles to the protuberances, fewer optical elements and fewer skid-resistant particles can be employed without sacrificing retroreflective performance and skid-resistance.
In the retroreflective element area, U.S. Pat. No. 3,418,896 discloses shaped polymeric retroreflective elements having a pigmented core and glass optical elements embedded in the vertical edges. These retroreflective elements are formed by extruding or otherwise molding the pigmented polymer into rods of different cross-sectional shape. Glass optical elements are embedded into the surface of the polymer before it hardens, then the rods are sliced to form the desired retroreflective elements. During the application step, the glass spheres are at the temperature of the extruded rods. This process is difficult to scale up because a hot, partially molten strand of core material is generally quite weak and tends to break during processing.
U.S. Pat. No. 5,822,120 (Palazotto et al.) discloses a retroreflective element comprising a core having a central layer and barrier layers applied to two major surfaces of the core layer, and a plurality of optical elements embedded in the other surfaces of the core layer. The retroreflective element can be made by extruding a central layer between the barrier layers, calendering to a desired thickness, processing into a desired shape and size, and then embedding the optical elements. The core of the retroreflective elements disclosed in Palazotto et al. typically is pigmented throughout to provide a system for retroreflection.
The need exists for a method of making retroreflective elements having optical elements and/or skid-resistance particles on selected surfaces and having enhanced retroreflection when wet and which provide delineation in dry and in wet conditions, and in low visibility conditions improving driver knowledge of vehicle position thereby increasing driver safety.